Room Persistence Library

The Room persistence library provides an abstraction layer over SQLite to allow fluent database access while harnessing the full power of SQLite.
The library helps you create a cache of your app's data on a device that's running your app. This cache, which serves as your app's single source of truth, allows users to view a consistent copy of key information within your app, regardless of whether users have an internet connection.

Components of RoomWe have 3 components they are
Entity : It’s nothing but a model class annotated with @Entity where all the variable will becomes column name for the table and name of the model class becomes name of the table.Database: This is an abstract class where you define all the entities that means all the tables that you want to create for that database.Dao: This is an interface which acts is an intermediary between the user and the database. All the operation to be performed on a table has to be defined here.

Sample Example is shown below

Following dependencies in build.gradle file

 // ViewModel and LiveData

implementation "android.arch.lifecycle:extensions:1.0.0"    

annotationProcessor "android.arch.lifecycle:compiler:1.0.0"
// Room    

implementation "android.arch.persistence.room:runtime:1.0.0"    

annotationProcessor "android.arch.persistence.room:compiler:1.0.0"
// Paging    

implementation "android.arch.paging:runtime:1.0.0-alpha4-1"
// Test helpers for LiveData    

testImplementation "android.arch.core:core-testing:1.0.0"

// Test helpers for Room    

testImplementation "android.arch.persistence.room:testing:1.0.0"

Java Classes

@Entity(tableName = "user")
public class User {

    public int getUid() {
        return uid;
    }

    public void setUid(int uid) {
        this.uid = uid;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @PrimaryKey(autoGenerate = true)
    private int uid;

    @ColumnInfo(name = "first_name")
    private String firstName;

    @ColumnInfo(name = "last_name")
    private String lastName;


}


@Dao
public interface  UserDao {

    @Query("SELECT * FROM user")
    List<User> getAll();

    @Query("SELECT * FROM user where first_name LIKE  :firstName AND last_name LIKE :lastName")
    User findByName(String firstName, String lastName);

    @Query("SELECT COUNT(*) from user")
    int countUsers();

    @Insert    
    void insertAll(User... users);

    @Delete    
    void delete(User user);
}


@Database(entities = {User.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {


    private static AppDatabase INSTANCE;

    public abstract UserDao userDao();

    public static AppDatabase getAppDatabase(Context context) {
        if (INSTANCE == null) {
            INSTANCE =
                    Room.databaseBuilder(context.getApplicationContext(), AppDatabase.class, "user-database")
                            // allow queries on the main thread.                            // Don't do this on a real app! See PersistenceBasicSample for an example.                            .allowMainThreadQueries()
                            .build();
        }
        return INSTANCE;
    }

    public static void destroyInstance() {
        INSTANCE = null;
    }

}


public class MainActivity extends AppCompatActivity {

    List<User> usersList;

    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Storing sample Data       // populateWithTestData(AppDatabase.getAppDatabase(MainActivity.this));        //Retrieving the data for specific user        User testUser = findUser(AppDatabase.getAppDatabase(MainActivity.this), "Abhi", "Sai");
        Toast.makeText(MainActivity.this, String.valueOf(testUser.getUid()), Toast.LENGTH_LONG).show();
        //Retrieving all users data        usersList = getUsersList(AppDatabase.getAppDatabase(MainActivity.this));
        for (User testUserItems : usersList) {
            Log.i("User ", testUserItems.getFirstName() + "  " + testUserItems.getLastName());
        }

    }

    private static void populateWithTestData(AppDatabase db) {
        User user = new User();
        user.setFirstName("Sanjay");
        user.setLastName("Shah");
        addUser(db, user);
        user.setFirstName("Abhi");
        user.setLastName("Sai");
        addUser(db, user);

    }


    private static User addUser(final AppDatabase db, User user) {
        db.userDao().insertAll(user);
        return user;
    }


    private static User findUser(final AppDatabase db, String fName, String lName) {
        User tUser;
        tUser = db.userDao().findByName(fName, lName);


        return tUser;
    }

    private static List<User> getUsersList(final AppDatabase db) {

        List<User> userList;
        userList = db.userDao().getAll();


        return userList;
    }
}